Note: Descriptions are shown in the official language in which they were submitted.
~2~93~8
l Method of Integrally Bonding a Radiation Shielding and
2 Thermally Conductive Gasket to a Surface and Article
Fabricated by Same
S Technical Field
7 The invention relates to the field of gaskets and, in
8 particular, to a gasket integrally bonded to a surface
9 during the molding and curing thereof.
11 Background Art
12
13 Gaskets are commonly used to assist in the shielding
14 of electrical components from electromagnetic radiation
(EMR). This shielding is extremely important to eliminate
16 cross-talk and spurious signals that would occur when
17 electrical components are densely packed together or when
18 the electrical components are subject to impinging EMR~
19 In a typical case, the electrical components are placed in
a metallic receptacle and a metallic cover which has a
21 conductive gasket attached to it by an adhesive layer is
22 fastened to the receptacle. The gasket is generally made
23 conductive by the inclusion of metallic micro-particles in
24 an elastomer during fabrication of the gasket.
26 These gaskets have, however, proved less than
27 satisfactory for good EMR shielding and thermal
2& conductivity. This was due basically to the fact that the
29 adhesive used to bond the gasket to the cover is a poor
electrical conductor, and a poor thermal conductor due to
31 its erratic application, and thus the electromagnetic
32 radiation is able to pass between the gasket and the cover
33 and enter the receptacle, while heat transfer is impeded.
34 In addition, in the case of metallic-coated glass
microballoons, since the microballoons are free to move in
36 the elastomer under vibration, the microballoons lose
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contact with one another and increase the resistivity of
the gasket, thereby resulting in decreased resistance to
EMR. Furthermore, the use of adhesives causes uncon-
trolled variations in gasket conductance and height, due
to the inability to apply them in a perfectly uniform
thickness. This results in improper closure both where
the gasket height is too low and where the gasket height
is too high due to insufficient local closure force to
properly deflect the gasket. Moreover, since the
gaskets are generally cut after curing to fit a particu-
lar cover shape or contour, the micro-particles are
exposed to air and proceed to corrode, resulting in a
general degradation of the properties of the gasket.
Such gaskets also degrade in their electrical properties
due to handling, stretching and elongation, due to the
failure of the adhesives over time, and due to handling,
shelf-life and pot-life (during application) adhesive
complications. Typical gaskets, their method of
manufacture, and the manufacture of the metallic micro-
particles are described in U.S. Patent Nos. 3,140,342,
3,194,860 and 3,202,488, respectively. Finally,
existing gaskets cannot be used as shields against
ultrahigh frequencies (>20 GHZ) and for micro-integrated
circuits where complicated gasket designs and tight
tolerances are essential.
Accordingly, it is a general object of the present
invention to provide an improved shielding gasket
suitable for use against electromagnetic radiation,
which can be integrally bonded to a surface for use
against electromagnetic radiation.
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Disclosure of the Invention
In accordance with one particular aspect of the
present invention, there is provided a method of making
a shielding gasket integral with a surface comprising
the steps of:
(a) preparing a gasket material consisting of a
mixture of an elastomer, a conductive filler, a catalyst
and a bonding agent;
tb) filling a mold with the gasket material;
(c) covering the mold with a surface chosen to be
bonded to the gasket material; and
(d) curing the gasket material in contact with the
surface to utilize the bonding agent to bond the gasket
material to the surface;
the mold having at least one groove therein to
receive the gasket material and at least one groove to
receive a mixture of an elastomer, a catalyst and a
bonding agent adapted to form an environmental gasket
bonded to the surface upon being cured.
B
lX1~3Z8
3a
In accordance with still another aspect of the
present invention, there is provided in combination, a
high-frequency electromagnetic-radiation-shielding
gasket that has been formed and cured in a mold and is
integral with a surface; the combination comprising:
a molded and cured gasket comprising a mi.xture of a
cured elastomer, an~electrically conductive filler and a
bonding agent; and
on one side of the molded gasket only, a surface
bonded to the molded gasket by the bonding agent;
the filler and bonding agent each being interdis-
persed throughout the molded and cured gasket.
A still further aspect of the present invention
provides in integral combination, a cover and a molded
gasket for blocking transmission of high-frequency
electromagnetic radiation; the combination being for use
with an enclosure having an opening to ~hich continuing
and repeatable access is desired, and the combination
comprising:
a solid conductive cover adapted for use in
covering such opening in such enclosure, and adapted for
repeated removal and reuse; and defining a surface; and
a molded gasket integrally bonded to the surface
without any intermediate adhesive layer, but not bonded
to such enclosure; and consisting of a mixture of at
least an elastomer and an electrica'ly conductive
filler;
the filler being dispersed throughout the gasket,
including the portion of the gasket that is in closest
proximity to the surface.
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3b
The present invention also provides, in another
aspect, in integral combinationl a cover and a molded
gasket Eor blocking transmission of high-frequency
electromagnetic radiation; the combination being for use
S with an enclosure having an opening to which continuir.g
and repeatable access is desired; and the combination
comprising:
a solid electrically conductive cover adapted for
use in covering such opening in such enclosure, and
adapted for repeated removal and reuse; and defining a
surface; and
a molded gasket bonded ts the surface but not to
such enclosure, and consisting of a mixture of at least
an elastomer and an electrically conductive filler, the
filler comprising electrically conductive particles that
are held in physical contact with the surface by the
elastomer.
In a still further aspect of the present invention,
there is provided a high-frequency electromagnetic-
radiation-shielding molded gasket integral with a
surface and comprising:
a molded gasket consisting of a mixture of an
elastomer, an electrically conductive filler and a
bonding agent; and
on one side of the gasket only, a surface bonded to
the gasket by the bonding agent during the curing of the
gasket.
In yet another aspect of the present invention,
there is provided in integral combination, an enclosure
and a gasket for blocking transmission of electromag-
netic radiation; the combination being for use with a
cover, and such enclosure having an opening to
A
~t21~3~
3c
which continuing and repeatable access is desired; and
the combination comprising:
a solid conductive enclosure defining an opening
and a surface generally surrounding the opening, and
adapted for use with such a cover for covering the
opening; and
a molded gasket integrally bonded to the surface
without any intermediate adhesive layer, but not bonded
to such cover; and consisting of a mixture of at least
an elastomer and an electrically conductive filler;
the filler being dispersed throughout the gasket,
including the portion of the gasket that is in closest
proximity to the surface.
In a further aspect of the present invention, there
is provided in integral combination, an enclosure and a
gasket for blocking transmission of electromagnetic
radiation; the combination being for use with a cover,
and such enclosure having an opening to which continuing
and repeatable access is desired; and the combination
comprising:
a solid conductive enclosure defining an opening
and a surface generally surrounding the opening, and
adapted for use with such a cover for covering the
opening; and
a molded gasket bonded to the surface but not to
such cover, and consisting of a mixture of at least an
elastomer and an electrically conductive filler, the
filler comprising electrically conductive particles that
are held in physical contact with the surface by the
elastomer.
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3d
Yet another embodiment of the present invention
provides a molded gasket, integral with a surface, made
by the process comprising the steps of:
(a) preparing a gasket material consisting of a
mixture of an elastomer, a conductive filler, a catalyst
and a bonding agent;
(b) filling a mold with the gasket material;
(c) covering the mold with a surface chosen to be
bonded to the gasket material; and
Id) curing the gasket material in contact with said
surface to utilize the bonding agent to bond the gasket
material to the surface.
Another aspect of the present invention provides a
high-frequency electromagnetic-radiation-shielding
molded and cured gasket for use with a closure surface
and comprising:
a molded and cured elastomer that has electrically
conductive filler dispersed in it and that is formed
into elongated shapes adapted for generally following
the edge of such a closure surface;
a bonding agent that significantly decreases the
compressibility of the gasket; and
longitudinal ribs defined in one side of the
elastomer;
whereby the filler provides effective electrical
conduction from the integral surface to such closure
surface, through the ribs, when the ribs and the closure
surface are pressed together; and
thereby minimizing the closure force required to
press the molded and cured gasket and the closure
surface together to provide such effective conduction,
despite the presence of such compressibility-decreasing
bonding agent.
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3e
Yet another embodiment oE the present invention
provides a metllod of making a compressible shielding
gasket integral with a mating surface, for repetitive
use and reuse in compression contact between the surface
and another closure surface; in the finished gasket
there being an electrically conductive filler dispersed
throughout an elastomeric matrix, the filler being
fixedly bonded in place to the elastomer; the method
comprising the steps of:
(a) preparing a mold as the reverse shape of a
gasket;
(b) preparing an elastomer of relatively high
compressibility;
(c) adding to the elastomer a catalyst;
(d) adding to the elastomer a conductive filler to
impart electrical conductivity to the elastomer;
(e) adding to the elastomer a bonding agent;
(f) mixing the elastomer, catalyst, conductive
filler and bonding agent to form a gasket material
throughout which the elastomer and the bonding agent are
in intimate contact with the filler and with one
another;
(g) then filling the mold with said gasket
material;
(h) covering the mold with such a mating surface
chosen to be bonded to the gasket material so that one
side only of the material contacts the surface; and
(i) then simultaneously:
~1) bonding, by virtue of the bonding agent,
the elastomer to the conductive filler throughout the
material,
(2) bonding, by virtue of the bonding agent,
the elastomer to the surface where the material is in
contact with the surface to form a unitary gasket and
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328
surface combination, at one side only of the material,
that is repetitively reusable in a compression contact
between the surface and such other closure surface; and
(3) curing the elastomer to form a free
surface of the gasket material where the material is in
contact with the mold to provide a gasket that is
repetitively reusable in a compression joint.
The novel features which are believed to be charac-
teristic of the invention, both as to the method and the
article fabricated thereby, together with further ob-
jects and advantages thereof, will be better understood
i;21~328
1 from the following description in connection with the
2 accompanying drawings in which a presently preferred
3 embodiment of the invention is illustrated by way of
4 example. It is to be expressly understood, however, that
the drawings are for purposes of illustration and
6 description only and are not intended as a definition of
7 the limits of the invention.
9 ~rief Description of the Drawings
11 Fig. 1 is a perspective view of a metallic shielding
12 box.
13
14 Fig. 2 is a cross-sectional view of the shielding box
lS of Fig. 1 taken along line 2-2 of Fig. 1 illustrating a
16 prior art gasket.
17
18 Fig. 3 is a detailed view of the region 3-3 of Fig. 2.
19,
Fig. 4 is a cross-sectional view of the shielding box
21 of Fig. 1 taken along line 4-4 of Fig. 1 illustrating the
22 gasket of the present invention.
23
24 ~ig. 5 is a detailed view of the region 5-5 of Fig.
4.
26
27 Fig. 6 i5 a plan view of a second embodiment of the
28 present invention.
29
Fig. 7 is a cross-sectional view of the embodiment of
31 Fig. 6 taken along line 7-7 of Fig. 6.
32
33 Fig. 8 i5 a cross-sectional view of the embodiment of
34 Fig. 6 taken along line 8-8 o$ Fig. 6.
36
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1 Best Hode for Carrying Out the Invention
3 Referring now to Fig. 1, a perspective view of a
4 metallic shielding box is illustrated. The box 10
comprises a metal cover 12 fastened to a metal receptacle
6 14 by means of screws 16. Between the cover 12 and the
7 receptacle 14 is a conductive gasket 18 which acts to
8 prevent electromagnetic radiation from entering the box 10
9 between the cover 12 and the receptacle 14. For purposes
of descriptive contrast, the gasket 18 is shown to consist
11 of two gasket portions, 18a and 18b, gasket portion 18a
12 being of prior art construction and gasket portion 18b
13 being constructed according to the present invention.
14
In Figs. 2 and 3, the prior art gasket portion 18a is
16 illustrated. The gasket portion 18a consists of an
17 elastomer 20 in which a conductive filler 22, such as a
18 plurality of metallic micro-particles, is embedded. The
19 filler 22 is mixed with the elastomer 20 when the
elastomer 20 is in an uncured flowable state and becomes
21 part of the structure of the gasket portion 18a when it is
22 cured to cause the gasket portion 18a to become
23 conductive. The gasket portion 18a can initially exist as
24 part of a larger sheet of material which is then cut to a
particular shape or contour, or it can be extruded or
26 molded. In the general case where the gasket portion 18a
27 is cut, a certain portion of the filler 22 is exposed
2& during the cutting process, as shown in Fig. 3, which
29 filler portion 22' is then susceptible to corrosion from
exposure to air and moisture and proceeds to cause
31 degradation of the entire gasket due to internal spread of
32 such corrosion. The gasket portion 18a has a layer of
33 adhesive 24 applied to the elastomer 20 which serves to
34 bond the gasket portion 18a to the cover 12. For the
reasons stated above, the adhesive 24 is a poor conductor
3~ and thus allows a portion of the impinging electromagnetic
1~19328
1 radiation to enter the box 10 between the cover 12 and the
2 ~elastomer 20. Since the filler 22 is poorly bonded to the
3 elastomer 20, in the case where the filler 22 consists of
4 metallic-coated glass microballoons, the microballoons are
free to move under vibration and thus lose contact with
6 one another, causing the gasket portion 18a to become less
7 conductive and to more freely transmit electromagnetic
8 radiation. Furthermore, general handling of the separate
9 gasket portion 18a and adhesive application of it to the
cover 12 causes elongation and loss of intimate contact of
11 the filler 22, causing it to become less conductive and to
12 more freely transmit electromagnetic radiation.
13
14 Referring now to Figs. 4 and S, the gasket portion
18b consists of a mixture of elastomer 26, bonding agent
16 28 and filler 30. As more fully described hereinafter,
17 the elastomer 26, the bonding agent 28 and the filler 30
18 are mixed with a catalyst when the elastomer 26 is in an
19 uncured flowable state and the uncured mixture is placed
in a mold which is sealed with the cover 12. Pressure and
21 heat are then applied to cure the mixture. The result is
22 a gasket portion 18b in which the bonding agent 28 bonds
23 the elastomer 26 into integral contact with the cover 12,
24 and since the bonding agent 28 also bonds the elastomer 26
to the particles of the filler 30, the filler 30 is held
26 in physical contact with the surface of the cover 12 by
27 the elastomer 26. In addition, the particles comprising
2& the filler 30 are bonded into place within the elastomer
29 26, are held in physical contact with one another by the
elastomer 26, and thus do not move under vibratory forces.
31 Also, since the gasket portion 18b is molded to a
32 particular shape or contour and is not cut, none of the
33 filler 30 is exposed to moisture or air. Finally, since
34 the gasket is cured and bonded in place, none of the EMR
shielding or thermal characteristics are degraded due to
36 handling, stretching, elongation and cutting of the
i21~328
1 gasket, and poor and uneven adhesive application and
2 general adhesive failure. It has been found, moreover, in
3 pull tests that the gasket of the present invention fails
4 internally before the gasket-cover interface fails. Thus
good thermal conductivity is obtained because of the
6 intimate filler-filler-cover contacts, none of the
7 impinging electromagnetic radiation can leak between the
8 cover 12 and the elastomer 26, as in the prior art
9 devices, and the volume resistivity of the gasket portion
18b, generally of the order of .004 to 10 ohm cm., remains
11 at a low fixed value undPr the action of vibratory forces
12 due to the bonding of the filler 30 to the elastomer 26.
13
14 The gasket portion 18b is made in accordance with the
method of the present invention by mixing 60% by volume of
16 an elastomer, such as a 10 durometer silicone rubber, with
17 40% by volume of a conductive filler, such as silver-
18 coated copper micro-particles having a diameter less than
19 6 microns. The particular percentages of the elastomer
and the filler and the diameter of the filler will,
21 however, vary depending upon the type of filler being used
22 and the desired resistivity of the gasket. Other fillers
23 could include silver and copper powder, particles or
24 spheres, silver-coated glass particles or spheres,
aluminum powder, particles or spheres, tungsten powder,
26 particles or spheres, nickel powder, particles or spheres,
27 iron powder, particles or spheres, aluminum powder,
28 particles or spheres, metallized coated ceramic powder,
29 particles or spheres and conductive carbon particles.
Mixtures of the foregoing may also be used. The type of
31 filler used is based upon environment, electrical needs
32 and mechanical configura~ions. Next, a bonding agent,
33 .5-2.0%, typically l~, by weight of the elastomer, is
34 milled into the above mixture when the filler is added. A
bonding agent which may be used in the process can be
36 triallylcynanurate or can be chosen from the class of
121~3~8
-- 8
1 silicate ~sters. It should be noted that wher~s in the
2 prior art mixtures the bonding agent is placed on the
3 metal surface to which the gasket is to be bonded, the raw
4 uncured elastomer in the present invention contains the
bonding agent and allows bonding of the elastomer to take
6 place to both the filler and the surface to be bonded to.
7 Finally, a catalyst, .5-5.0%, typically 1%, by weight of
8 the elastomer, is added to the mixture. The catalysts
9 used in the process may be of a peroxide base, such as
10 dicumyl peroxide; or the commercially available material
11 known ~y the ~rade name "Varox", which is 2,5-dimethyl-2,
12 5-bi~ (t-butyl-peroxy)hexane and is made by the R. T.
13 Vanderbilt Company; or the commercially available material
14 known by the trade name "BSG", which is benzoyl peroxide and
is made by Thalco; such catalysts operate in a ~emperature
16 range about 360, 310F and 235 F, respectively. The
17 catalyst is chosen so as to be compatible with materials used
18 in conjunction with the cover 12, such as paints, metal
19 finishes, plastics, etc. Other catalysts, such as platinum
based on catalysts, can be used where reversion of the
21 elastomer, both mechanical and chemical, is of prime concern.
22 The mixture is then calendered to a selected
23 thickness, cut into strips and inserted into a mold having
24 a reverse-imzge slot machined therein of the desired
gasket configuration. The mating surface, such as a
26 metallic cover, is placed on the mold. The surface can be
27 selected from a variety of metals and can be treated with
28 various finishes, such as chem film, electrolysis nickel,
29 o~ tin or silver plated, etc. A press then applies a
pressure of 300 PSIG for 15 minutes at 310F to cure,
31 confi~ure and hot vulcanize the mixture in contact with
32 the mating surface. The mold is then removed from the
33 press, left to cool to room temperature, and the flash
34 tri~med away. The temperature chosen may range from
360F-240F depending upon the particular catalyst used.
36 The time will also vary depending upon the thickness of
~219328
1 -the gasket being cured, with 15 minutes being suitable for
2 a gasket .08~ in thickness.
4 Using the above method, a multiribbed gasket can be
fabricated which has been found to be extremely effective
6 against electromagnetic radiation and which, if desired,
7 can be combined with an environmental seal, as described
8 below. As shown in Figs. 6, 7 and 8 the gasket 32 has a
9 plurality of ribs 34, 36 which are bonded to a metallic
mating surface 38 using the above-described method. The
11 ribs 34 are first formed in accordance with the above-
12 described method using a multiribbed mold similar to the
13 single-slot mold used in the above-described method but
14 having a selected number of grooves, generally parallel,
machined therein for formation of the ribs 34. The ribs -
16 34 generally range from 2-7 in number and are, for
17 example, from .022~ to .050~ in depth depending on the use
18 of the gasket, with the gasket ranging from .032~ to .060
19 inches in thickness in consonance with the depth of the
ribs. The ribs 34 are .035~ thick with an .035" spacing
21 ~etween them and the width of the ribs 34 ranges from
22 .251~ to .601~ depending upon the number of the ribs 34.
23 It can readily be seen that since the rib dimensions and
24 configurations are solely a function of the tooling, rib
dimensions and configurations can now be obtained that
26 were heretofore desirable but impossible.
27
2& A mixture of elastomer, bonding agent and catalyst,
29 similar to that used for the ribs 34 but excluding the
filler, is then used for the fabrication of the ribs 36
31 which comprise the environmental seal. The elastomer may
32 consist of a 20-30 durometer fluorosilicone rubber. The
33 multiribbed mold used for the ribs 34 also contains the
34 grooves for the ribs 36, which grooves are generally
parallel to the ribs 34. After the ribs 34 have been
36 fabricated, the grooves for the ribs 36 are then filled
~19328
1 ~ith the above-recited mixture, the surface 38 with the
2 ribs 34 thereon is placed again on the mold and heat and
3 pressure are applied to cure, configure and hot vulcanize
4 the ribs 36 to the surface 38. The ribs 36 range in depth
from .052" to .083", are typically .063~ in width and are
6 spaced .010U from the ribs 34. Since the tooling can be
7 configured so that both the environmental seal and the
8 shielding gasket can be configured, cured and bonded at
9 the same time, the ribs 36 can also be fabricated
concurrently with the ribs 34 if the processing times,
11 temperatures and pressures are compatible for the two
12 mixtures. If there are any holes in the surface 38, such
13 as hole 40, the ribs 34, 36 can be configured to curve
14 around the holes. If the holes are small enough, such as
hole 42, the outer ribs 44 of the ribs 34 are made to pass
16 on either side of the outside of the hole 42, while the
17 inner ribs 46 of the ribs 34 are made to terminate on
18 either side of the centers of the hole 42. A compression
19 washer or stand-off 48 is formed around the hole 42 during
the fabrication of the surface 3B, which washer 48 is
21 integral with and of the same material as the surface 38
22 and prevents compression of the ribs 34, 36. In order to
23 form this configuration~ the surface can be machined to
24 form compression washers around the holes in the surface,
and the mold can be machined to have parallel grooves
26 corresponding to the ribs, some of which would tèrminate
27 at the compression washers and others of which~would pass
28 by the outer edges of the compression washers.
29
A novel method of making a radiation shielding and
31 thermally conductive gasket integral with a surface has
32 thus been described which permits the manufacture of
33 shielding gaskets in dimensions and configurations and for
34 applications heretofore deemed impossible, and which can
vastly improve gasket EHR shielding and thermal
36 conductivity in current applications. With this in mind,
328
11
1 it is obvious that numerous modifications and departures
2 may be made to the present invention by those skilled in
3 the art; thus the invention is to be construed as being
4 limited only by the spirit and scope of the appended
claims.
7 Industrial Applicability
9 The radiation shielding and thermally conductive
gasket integral with a surface, and the method of
11 fabrication thereof, is useful for shielding against
12 electromagnetic radiation, provides good thermal
13 conductivity and, in conjunction with an environmental
14 seal, affords environmental protection.
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